Long, Andrew J.Schiappacasse, Enrico D.2025-01-092025-01-092024Long, A. J., & Schiappacasse, E. D. (2024). Resonant conversion of axion dark radiation into terahertz electromagnetic radiation in a neutron star magnetosphere. Physical Review D, 110(10), 103020. https://doi.org/10.1103/PhysRevD.110.103020https://hdl.handle.net/1911/118112In the strong magnetic field of a neutron star’s magnetosphere, axions coupled to electromagnetism develop a nonzero probability to convert into photons. Past studies have revealed that the axion-photon conversion can be resonantly enhanced. We recognize that the axion-photon resonance admits two parametrically distinct resonant solutions, which we call the mass-matched resonance and the Euler-Heisenberg assisted resonance. The mass-matched resonance occurs at a point in the magnetosphere where the radially-varying plasma frequency crosses the axion mass 𝜔pl ≈𝑚𝑎. The Euler-Heisenberg assisted resonance occurs where the axion energy satisfies 𝜔 ≈(2⁢𝜔2pl/7⁢𝑔𝛾⁢𝛾⁢𝛾⁢𝛾⁡¯𝐵2)1/2. This second resonance is made possible though the strong background magnetic field ¯𝐵, as well as the nonzero Euler-Heisenberg four-photon self-interaction, which has the coupling 𝑔𝛾⁢𝛾⁢𝛾⁢𝛾 =8⁢𝛼2/45⁢𝑚4𝑒. We study the resonant conversion of relativistic axion dark radiation into photons via the Euler-Heisenberg assisted resonance, and we calculate the expected electromagnetic radiation assuming different values for the axion-photon coupling 𝑔𝑎⁢𝛾⁢𝛾 and different amplitudes for the axion flux onto the neutron star Φ𝑎. We briefly discuss several possible sources of axion dark radiation. Achieving a sufficiently strong axion flux to induce a detectable electromagnetic signal seems unlikely.engExcept where otherwise noted, this work is licensed under a Creative Commons Attribution (CC BY) license. Permission to reuse, publish, or reproduce the work beyond the terms of the license or beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Journal articlePhysRevD-110-103020https://doi.org/10.1103/PhysRevD.110.103020